Add dontwarn flags for android.support

Change-Id: Ic67a63061a8c9a5882103892555fd940aa63bb6f
diff --git a/proguard.flags b/proguard.flags
index 75fbfd7..07ec5d2 100644
--- a/proguard.flags
+++ b/proguard.flags
@@ -2,6 +2,11 @@
   public <init>(...);
 }
 
+# The support library contains references to newer platform versions.
+# Don't warn about those in case this app is linking against an older
+# platform version.  We know about them, and they are safe.
+-dontwarn android.support.**
+
 # Xml files containing onClick (menus and layouts) require that proguard not
 # remove their handlers.
 -keepclassmembers class * extends android.app.Activity {